The present invention relates to turbochargers and, more particularly, to turbochargers having adjustable vanes which can vary the exhaust gas flow to the turbine portion of the turbocharger so as to vary the output power of the turbine portion.
Turbochargers are well known devices which utilize the energy of exhaust gases from an internal combustion engine to compress combustion air flowing to the combustion chambers of the engine. Briefly, a turbocharger comprises two impellers mounted on opposite ends of a common shaft, each impeller capable of rotating within its own cavity within the turbocharger housing. One impeller functions as a fluid motor, the exhaust gases from the engine causing rotation of the impeller. At the other end of the common shaft, the other impeller, commonly termed the pump or compressor impeller, functions to draw in ambient air and td compress the air to higher pressure which can be used, for example, to increase the flow of combustion air into the engine to thereby increase engine power.
Thus, in this use, the turbocharger functions as an air mass flow control for the engine. As a consequence, the turbocharger must be designed in terms of impeller volutes and impeller blade orientation to best match the requirements of the engine over its entire range of speeds. With a conventional turbocharger of a fixed geometry design, such a match will necessarily be a compromise of the best performance possible at various engine speeds and torques. For example, if the turbocharger is designed so as to provide to the optimum air flow at maximum engine speed, the flow will be less than optimum at lower engine operating speeds and vice versa.
Furthermore, after the engine and turbocharger are operated for a period of time, wear and dirt accumulation can change the operating characteristics of one or both of the engine and turbocharger and thus the compromise match between the two components may change even further to the detriment of engine performance. The problem of matching the turbocharger with the engine is also compounded by the fact that, in a large scale manufacturing operation, there may be differences from one engine to another and from one turbocharger to another due to manufacturing tolerances. In view of the more stringent requirements for fuel economy and emissions which are forthcoming for motor vehicles, it would be highly desirable to provide a turbocharger which could match the engine over a wide range of operating conditions.
It has been long recognized in the turbocharger art that if the power of the turbine portion could be varied by a suitable control, one could precisely control the airflow to the engine at any engine speed and torque. In addition, with such a control, the airflow to the engine could be modified during transient power changes thus reducing so-called "turbo lag" and reducing particulate emissions. Furthermore, a turbocharger with a variable power turbine portion could compensate for changes in the engine or the turbocharger itself caused by wear and the accumulation of dirt or other foreign matter.
Such turbochargers having a variable power turbine are shown in, for example, U.S. Pat. No. 2,428,830 to Birmann and in U.S. Pat. No. 3,945,762 to Leicht. Despite the potential advantages of such turbochargers in enabling the turbocharger air output to be controlled to some extent, they have not achieved a significant penetration in the commericial turbocharger market. This is due, at least in part, to the inability to precisely control the turbocharger output, and the mechanical difficulties encountered in providing a variable power turbocharger which will withstand prolonged use.